For example, as the patterns formed on semiconductor wafers are miniaturized, dimension measurement and defect detection by optical measurement and inspections conventionally performed for process control of the semiconductors have become more and more difficult. Therefore, as apparatuses using charged particle beams, measurement and inspection apparatuses using scanning electron microscopes (hereinafter, abbreviated as SEM) have been put into practical use. These are CD-SEM (Critical Dimension-SEM) apparatuses which measure the line widths of fine patterns and hole diameters for condition setting or monitoring of manufacturing processes of semiconductors and SEM inspection apparatuses which detect the defects by pattern comparison of electron beam images.
In such CD-SEM apparatuses or SEM inspection apparatuses, if the focal position of an optical system of the SEM thereof is defocused, pattern widths and hole diameters are varied and correct measurement cannot be performed, or in defect detection by pattern comparison, normal parts may be erroneously detected as defects. Therefore, focusing is important. The focusing is achieved by using an electron beam image through the adjustment of an excitation current of an objective lens. Meanwhile, in automatic focus control which automatically performs the focusing, the focal position of an objective lens is changed, an electron beam image is detected at each position, and a focused focal point is detected from the electron beam images. On the other hand, as an optical method of the focusing, there is a method in which a wafer is irradiated with slit light from a position obliquely above it, the reflected light thereof is detected, the height of the wafer is obtained from the detected position of the reflected light, and then the focusing of an objective lens is performed according to the obtained height. When magnification of a SEM is low, the focusing of an objective lens can be achieved by the optically obtained height of a wafer. However, since the focal depth of the SEM is reduced when the magnification of the SEM is high, the focusing of the objective lens according to the optically obtained height of the wafer is difficult in terms of accuracy. Therefore, automatic focus control is performed.
In general, since a plurality of electron beam images are obtained in the automatic focus control, the processing time thereof is longer than the method which optically detects the wafer height. Therefore, if the optical wafer height detection can be made more accurate, the processing time of the automatic focus control performed based on it can be shortened. In the case of a CD-SEM apparatus, since optical height detection and automatic focus control based on it are performed, the focus control takes up long time in a measurement process. Accordingly, the throughput of the CD-SEM can be improved by shortening the time of the focus control. Furthermore, in a SEM inspection apparatus, focal position control is performed by optical height detection, electron beam images are successively obtained, and a comparison test between dies or cells of the wafer is performed. Therefore, since the optical height detection can be performed at high accuracy, high-quality electron beam images of focused focal positions are obtained, and highly reliable inspection can be performed.
As optical height detection methods, Japanese Patent Application Laid-Open Publication No. 11-183154 and Japanese Patent Application Laid-Open Publication No. 11-149895 disclose the methods for an electron-beam-type measurement apparatus or an electron-beam-type inspection apparatus, in which a multi-slit-like flux of light is projected to a wafer from a point obliquely above it, the multi-slit-like flux of light reflected from the surface of the wafer is caused to form an image, image formation states having fewer detection errors are searched and subjected to a weighting process, the height of the wafer is obtained from a movement distance with respect to a reference height, and a focal point of an objective lens of an electron optical system is focused according to the obtained height.
Japanese Patent Application Laid-Open Publication No. 2002-203785 discloses the method for an exposure apparatus, in which a plurality of minute light fluxes are caused to enter a wafer surface from a light irradiation means disposed at a position obliquely above it, a plurality of reflected light fluxes from a pattern region are detected, the plane position information of the fine pattern region is detected from obtained signals to obtain the height of the wafer, and the fine pattern region is adjusted to a focal position of the exposure apparatus.